Variable Interest Entities (Details 6) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of net income (loss) attributable to the entity and transfers to the non-controlling interest
Net income attributable to the Partnership's unitholders	$ 38,077,000	$ 220,402,000	$ 60,695,000
Transfers to the non-controlling interests:
Decrease in common unit equity for 2011 acquisition of equity interest in MarkWest Liberty Midstream, net of $51,321 income tax benefit			(1,194,865,000)
Tax benefit resulting from change in equity due to purchase of additional interest in consolidated subsidiary			51,321,000
Decrease in common unit equity for transaction costs related to 2011 acquisition of equity interest in MarkWest Liberty Midstream			(3,600,000)
Net (loss) income attributable to the Partnership and transfers to the non-controlling interest			(1,137,770,000)
MarkWest Liberty Midstream
Variable Interest Entities
Percentage of non-controlling interest acquired by the Partnership			49.00%
Non-controlling interest purchase consideration, cash paid			$ 994,000,000
Issuance of Class B units (in units)			19,954,000